The proposed research includes two separate projects: one on neuronal encoding mechanisms, and a second on transmitter-receptor interactions of the postjunctional membrane and the resulting alteration of membrane permeability. Many neurons encode information as trains of nerve impulses. The essential features of this encoding process have recently become clear, but certain slowly varying properties of the encoder (e.g., adaptation) are not yet well understood. By extending the Hodgkin-Huxley type analysis previously developed in this laboratory, these slow processes will be included in the present model for the encoder. These experiments should lead to a complete specification of the encoding properties in one neuron type. Recently certain properties of the postjunctional membrane have been accounted for by a model of acetylcholine-receptor interactions based upon current view of enzyme-substrate interactions: acetylcholine is thought to bind rapidly to its receptor and to induce a conformational change in the receptor which is responsible for altered membrane permeability. Quantitative predictions, derived from this model, about the fluctuations in the number of receptor-acetylcholine complexes in each conformational state will be tested by making measurements on the random fluctuations in membrane permeability.